It is known to provide a linear hydraulic actuator, having a piston reciprocable in a cylinder including fluid ports at its ends beyond the travel range of the piston, with a stop member adjustable along the piston rod projecting from one end of the cylinder, a valve operable to prevent egress of fluid from the other end of the cylinder, and means such as a push rod for enabling the stop member to operate the valve as the piston is retracted into the cylinder, thus hydraulically locking the actuator against further retraction of the piston.